1. Field of the Invention
The present invention relates to an apparatus for welding joint sections of straps and others in a supporting grid, which constitute a part of a nuclear fuel assembly for an atomic reactor.
2. Description of the Related Art
Conventionally, a nuclear fuel assembly used in light water atomic reactors, for example, is comprised by a plurality of supporting grids disposed between an upper nozzle and a lower nozzle that are separated at a given distance, and includes an instrument pipe and a plurality of control rod guide pipe fixed to each supporting grid, upper nozzle and lower nozzle, and fuel rods inserted in the grid spaces of each supporting grid. Each supporting grid is constructed by cross joining the straps made of thin strips in a lattice form.
For example, in the supporting grid 1 shown in FIG. 5, individual inner straps 2 are assembled to form a lattice structure containing a plurality of grid spaces 3, and the joint sections (weld sections) P formed by crossed straps 2 are spot welded using a laser welding apparatus and the like. Further, a sleeve 4 is inserted in a portion of the grid space 3, and the abutting sections Q formed at the cutout section 5 formed at the top and bottom ends of the inner strap 2 are seam welded in a continuous line by using a laser welder. A control rod guide pipe 6 is inserted in each sleeve 4, and the two are locked as a unit by expanding the tubes.
Also, at the intersection points of the inner straps 2 and the outer straps 8 shown in FIG. 6, weld tabs 7 formed on both ends of each inner strap 2 are coupled to respective grooves 8a of the outer straps 8, which are disposed on the four corners of the supporting grid 1, to form a coupling section R (welding section), which is also welded.
The supporting grid 1 thus assembled has an upper surface 1a and a lower surface 1b of a square shape, having many sections to be welded, such as a plurality of joint sections P and butting sections Q, and a rectangular shaped side surfaces 1c having the coupling sections R. Therefore, there is a need to carry out a large number of welding operations in making a fuel assembly.
Welding of supporting grid 1 mast be carried out in an atmosphere containing less than a certain defined level of oxygen and moisture, therefore, welding operations must be carried out in a hermetically sealed welding chamber, and furthermore, because the fumes generated by welding must be exhausted, it is necessary to constantly supply an inert gas into the welding chamber. Welding operations inside the chamber are carried out by using a grid driving device to move the supporting grid 1 inside the chamber to weld the sections such as the joint sections P and the butting sections Q successively, because so many sections to be welded, such as the joint sections P and the butting sections Q, are involved.
However, because of a large quantity of fumes generated during the welding process of so many welding sections of the supporting grid 1, residual fumes inside the chamber, and attach themselves to the glass surfaces that are protecting the laser emitting head from fogging the glass surface, resulting in a problem of lowering the transmission efficiency of laser power through the glass. For this reason, it has been necessary to introduce an inert gas into the chamber and sweep the fumes away with the inert gas externally.
However, in the conventional supporting grid welding apparatus, the grid drive device for moving the supporting grid 1 is located inside the chamber, resulting in a large size for the facility and a corresponding need for a large volume of inert gas as well as a lengthy gas filling process and high cost.
It is an object of the present invention to provide a grid welding chamber so that welding operations can be carried out efficiently by making the chamber more compact.
The object has been achieved in a supporting grid welding apparatus having a feature that a laser device and a grid drive device are disposed outside the welding chamber, and the grid moving device is used to move the supporting grid relative to the supporting grid.
According to the present supporting grid welding apparatus, the chamber is required to house the supporting grid and it is only necessary to move the chamber slightly relative to the supporting grid. Therefore, compared with the conventional welding apparatus having the grid drive device inside the chamber, the present welding chamber can be made smaller in size, resulting in reducing the time of filling the chamber and the volume of gas required to replace the chamber atmosphere with an inert gas, thus improving the work efficiency and lowering the cost.
When performing welding, the laser device is moved relative to the chamber and the supporting grid so as to position the welding section of the supporting grid at the focal point of the laser beam, and the chamber is moved relative to the laser device and the supporting grid so that the welding section of the supporting grid will be distanced away from the surface through which the laser beam is transmitted.
When performing welding using a high power output from the laser device, such as for spot welding, if the distance between the welding section of the supporting grid and the chamber surface is too short, a plasm gas generated from welding operations can strike the chamber surface (glass) severely to cause thermal damage to the glass, or the fume may adhere to the glass surface, so that it is necessary to move the welding section of the supporting grid away from the glass surface through which the laser beam is transmitted.
It is preferable that the chamber surface, at least in those regions, where the laser beam is to be transmitted, to be constructed of a material transmissive to the laser light. Also, the chamber interior may be filled with an inert gas.
Fumes produced from welding operations can adhere to the chamber surface, resulting in reducing the power of the laser beam transmitted through the chamber surface. Therefore, it is necessary that the fumes remaining inside the chamber be swept with the inert gas to prevent reducing the welding efficiency. According to the present invention, fumes can be eliminated from the chamber efficiently because of the small size of the welding chamber.